1. Field of the Invention
This invention relates to fuel injection system of a gas turbine combustor and more specifically, to a method for balancing fuel flow to a plurality of fuel nozzles and to a plurality of combustor end covers about a gas turbine engine.
2. Description of Related Art
The U.S. Government has enacted requirements for lowering pollution emissions from gas turbine combustion engines, especially nitrogen oxide (NOx) and carbon monoxide (CO). These emissions are of particular concern for land based gas turbine engines that are used to generate electricity since these types of engines usually operate continuously and therefore emit steady amounts of NOx and CO. A variety of measures have been taken to reduce NOx and CO emissions including the use of catalysts, burning cleaner fuels such as natural gas, and improving combustion system efficiency. One of the more significant enhancements to land based gas turbine combustion technology has been the use of premixing fuel and compressed air prior to combustion. The resulting mixture is lean with respect to fuel concentration, and in some systems, can approach the lean flammability limit, or the point where combustion can no longer occur. When the resulting mixture does react and combustion occurs, a lower flame temperature results and lower levels of NOx are produced.
Since this type of combustion system operates close to the lean flammability limit, combustion is not as stable as the more typical diffusion flame combustion systems. Combustion instabilities are produced by oscillations in the combustor pressure field and can vary based on a variety of factors including fuel pressure, air pressure, turbine effects, and communication with adjacent combustors. These pressure oscillations, if not minimized and adequately controlled, can affect the integrity and life of the combustor.
Another issue of concern to operators of premix combustion systems is the effect of prolonged combustor wall exposure to elevated temperatures, especially non-uniform temperature distribution. Typically premix combustion systems contain a plurality of fuel nozzles for injecting fuel and mixing it with compressed air within a single combustor. More specifically, for land based gas turbine engines that are utilized to generate electricity, a plurality of combustion systems are typically required. Therefore, given the large amount of fuel nozzles on a given engine, a wide variety of fuel/air mixtures and flame temperatures are possible for each region of the combustor. If each fuel nozzle is injecting a slightly different amount of fuel when combustion occurs, the flame temperature will vary circumferentially about the combustor, depending on the mixture quality and fuel/air ratio for that particular region. That is, the more fuel rich regions will tend to have a higher flame temperature than leaner regions of the combustor. Also, the more poorly mixed regions will tend to not burn as completely as the better mixed regions, resulting in higher NOx emissions. This flame temperature fluctuation or variance will result in uneven temperature exposure to the combustor walls, which can lead to accelerated degradation requiring more frequent repairs and premature replacement of combustion components.
In an effort to control fuel flow and resulting combustion dynamics, prior art gas turbine combustors have utilized restriction devices such as meterplates. An example of this technology is shown in FIG. 1 and discussed in detail in U.S. Pat. No. 5,211,004. In practical use, a plurality of fuel nozzles are arranged about a housing 32 having a fitting 40 with a plurality of bores 52 serving to form a meterplate. The fuel nozzles in question may inject natural gas, liquid fuel, or both. Gas from annulus 50 is restricted by bores 52 before filling chamber 54 and being injected into a combustion chamber through orifice 56. Chamber 54 is filled with gas at a predetermined pressure, as a result of fitting 40 and bores 52, sufficient to supply a steady flow of fuel to compensate for pressure fluctuations in the combustion chamber. While this metering technology claims to reduce combustion dynamics, the plurality of fuel nozzles about housing 32 do not have balanced fuel flow rates relative to the supply of fuel to housing 32 and therefore still subject the combustion chamber to flame temperature fluctuations resulting in localized regions of distress. What is needed for a combustion system, which contains a plurality of fuel nozzles, is a configuration that not only helps to minimize combustion pressure oscillations and combustion dynamics but also to reduce flame temperature variations, such that the combustion chamber walls are exposed to a more uniform temperature distribution. The present invention accomplishes each of these tasks by disclosing a method of arranging nozzles about a combustor end cover as well as arranging a plurality of end covers about a gas turbine engine in order to provide a generally consistent fluid flow to each combustor while minimizing flame temperature variation and reducing overall emissions.